The present invention relates to micromachined devices used for measuring angular rate and acceleration. More particularly, the present invention relates to a triaxial angular rate and acceleration sensor for use in inertial measurement units that may be micromachined from a single substrate.
Inertial measurement units (IMUs) are critical to the proper operation of inertial navigation and guidance systems. Such systems are used on ships, aircraft, spacecraft, etc.
A typical IMU utilizes a cluster of three accelerometers and three gyros mounted to a structure which is shock isolated. The three accelerometers are used to measure linear acceleration while the gyros are used to measure angular rate.
U.S. Pat. No. 4,920,801, the mathematical equations of which are hereby incorporated by reference, is directed to a monolithic accelerometer capable of sensing linear acceleration in three orthogonal directions. The accelerometer utilizes three co-planar cantilever beams. A mass is formed on each of the cantilever beams. The mass is asymmetrically arranged about the neutral plane of the beam such that the sensing axes passes through the mass at an angle with respect to the plane of the beams. The beams are arranged at 120 degrees with respect to one another such that the sensing axes are substantially orthogonal. Although this accelerometer may be used to measure linear acceleration, it cannot measure angular rate.
Sundstrand Data Corporation has pioneered the development of a Single Coriolis Inertial Rate and Acceleration Sensor (SCIRAS(trademark)). In an article by Rand Hulsing II entitled xe2x80x9cSingle Coriolis Inertial Rate and Acceleration Sensorxe2x80x9d, Journal of the Institute of Navigation, Vol. 35, No. 3, pp. 347-59 (Fall 1988), the inventor describes a proof-of-concept mechanism which was capable of simultaneously measuring both linear acceleration and angular rate with the same accelerometer structure. The mechanism utilized two back-to-back linear accelerometers disposed on opposite sides of a flexible parallelogram structure. The parallelogram structure was rocked at a predetermined dither frequency. As the parallelogram is rocked about flexures at its corners, a predominantly linear motion is applied to both accelerometers in equal and opposite directions. Using the difference between the two accelerometer outputs, the linear component is measure. Using the sum of the two outputs, the linear components cancel, and only the Coriolis components remain. Thus the small Coriolis acceleration signal associated with angular rate can be extracted from a large linear vibration by matching the scale factors of the two accelerometers.
The present invention is directed to a monolithic sensor substrates which are adapted for use in sensors which allow simultaneous measurement of both linear acceleration and angular rate along three skewed axes. The sensor includes two sets of three accelerometers each. Thus a total of six accelerometers are used.
In accordance with one feature of the invention, the accelerometers are formed as a monolithic structure from a single substrate such as silicon. Each accelerometer includes a proof mass connected to a sensor frame by at least one flexure. All six accelerometers are arranged in a single plane. The input axis of each accelerometer is canted at an angle with respect to the plane. A first set of three accelerometers are disposed in the plane such that their input axes are skewed to one another. The remaining second set of three accelerometers are arranged such that their input axes are also skewed with one another and opposite in direction to the input axes of the first set of three accelerometers. The sensor frames of each accelerometer of the first set of accelerometers is connected to a corresponding accelerometer from the second set of accelerometers. The link allows the corresponding accelerometers to dither at the same frequency and further ensures that a force imparted to one accelerometer of the pair along the dither axis of the pair is also imparted to the corresponding accelerometer in an equal but opposite direction. All of the accelerometers in the plane are further mechanically linked to one another such that a single dither oscillator may be used to dither the accelerometers in the plane at the same dither frequency.
In accordance with another feature of the invention, the input axis of each accelerometer is canted by adding a mass plate as part of the proof mass to adjust the center of mass of the proof mass. The full scale acceleration range and the Q of the accelerometer can be set to a particular value dependent upon the density of the mass plate material.
In accordance with a further feature of the invention, the sensor may be designed to prevent angular acceleration sensitivity. In one embodiment of such a design, the first and second sets of accelerometers are not coplanar. Rather, the first set of accelerometers lie in a first plane while the second set of accelerometers lie in a second plane that is generally parallel to the first plane. The input axes of the first set of accelerometers are aligned with the input axes of the second set of accelerometers. The first set of accelerometers are linked to one another such that they dither at the same frequency. Likewise, the second set of accelerometers are linked to one another such that they dither at the same frequency. The first and second sets of accelerometers, in turn, are linked to one another such that they dither at the same frequency but with a phase differential.
The phase difference may be provided in several manners. In one particular embodiment, the phase difference is the result of a linking member associated with each one of the first set of accelerometers. While the second set of accelerometers undergo a dither force in a first direction, the linking member causes a counter force to be applied to dither the first set of accelerometers in the opposite direction. In a further embodiment, the phase difference is merely the result of the natural motion of the accelerometers. In a still further embodiment, a connection link is disposed between the substrate forming the first set of accelerometers and the substrate forming the second set of accelerometers. The connection link causes the first and second set of accelerometers to dither at the same frequency, but at a phase difference approaching 180 degrees.